Photosensitive media processors, such as Kodak X-OMAT processors, are useful in applications like the automatic processing of radiographic films for medical imaging purposes. The processors automatically transport sheets or rolls of photosensitive film, paper or the like (hereafter "film") from a feed end of a film transport path, through a sequence of chemical processing tanks in which the film is developed, fixed, and washed, and then through a dryer to a discharge or receiving end. The processor typically has a fixed film path length, so final image quality depends on factors including the composition and temperature of the processing chemicals (the processor "chemistry"), and the film transport speed (which determines the length of time the film is in contact with the chemistry).
In a typical automatic processor of the type to which the invention relates, film transport speed is set at a constant rate and the chemistry is defined according to a preset recommended temperature, e.g. 94.degree. F. (34.degree. C.), with a specified tolerance range of +/- X.degree.. A temperature control system is provided to keep the chemicals within the specified range, and means is provided for automatically replenishing the chemicals as they are used up.
Some processors use a thermowell located in a developer recirculation path to maintain a desired recommended developer chemical temperature. The thermowell has a cartridge heater inserted into one end of a hollow tubular body through which the developer is caused to flow by means of a pump. A thermistor protruding into the thermowell flow path serves to monitor the recirculating developer temperature. The duty cycle of the heater is varied, based upon data received from the thermistor, in proportion to the proximity of the measured actual temperature to a preestablished developer setpoint temperature. Until the setpoint temperature is reached, a "wait" light or similar annunciator signals the user that an undertemperature condition exists. Once the setpoint temperature is reached, heating and cooling cycles are initiated, as needed, in accordance with detected temperature variations from the setpoint. Cooling may be accomplished by operation of a solenoid valve which redirects the developer through a loop in the recirculation path which is in heat exchange relationship with cooler water in the wash tank. The fixer, whose temperature is less critical, may have its own thermowell recirculation path or may be maintained at a temperature close to the developer temperature by directing it in heat exchange relationship with the developer.
Processors have been introduced which are settable as to transport speed and chemistry temperature, so that the same processor can be used for multiple processing modes. A particular mode is often referred to by a shorthand designation indicative of its associated "drop time," which corresponds to the time lapse from entry of the leading edge of a film at the feed end of the processor, until exit of the trailing edge of the same film at the discharge end. Kodak uses the designations "Kwik" or "K/RA," "Rapid," "Standard," and "Extended" to refer to different user-selectable operating modes, each of which has its own characteristic transport speed and developer setpoint temperature.
The operations and functions of automatic film processors are handled under control of electronic circuitry, including a microprocessor connected to various process sensors and subsidiary controls to receive and dispense electronic signals in accordance with predefined software program instructions. Examples of such control circuitry are shown in U.S. Pat. No. 4,300,828 and in U.S. patent application Ser. No. 07/494,647. U.S. patent application Ser. No. 07/738,664, entitled "Method and Apparatus for Out-of-Rate Error Detection In Film Processor Temperature control system Jul. 31, 1991, describes a processor temperature control system in which malfunctions in operation of heating and cooling cycles are determined utilizing comparisons of actual and normal rates of change in chemical or dryer air temperature over time. U.S. patent application Ser. No. 07/759,484, entitled "Method for Detecting of Non-Valid States In a Film Processor Temperature Control System," filed on even date herewith, describes a method for verifying the validity of temperature measurement data based on comparisons of the measured actual temperatures of chemical with predictions as to what valid actual temperature states of the chemicals could be, given the heat gains (or losses) applied in the system during the time interval between measurements. The disclosures of those patent references are incorporated herein by reference.
In a typical processor of the type to which the invention relates, replenishment of developer or fixer chemical occurs automatically after a predetermined area of film has passed through the processor, and in response to a low level indicated by a chemical level sensor. Replenishment pumps are energized to introduce a slug of fresh developer or fixer from an external source of replenisher chemical. Because the external replenisher source is usually maintained at room temperature and it takes time for the newly introduced slug to mix with the chemical already in the tank, this presents problems for a temperature control system that utilizes a proportional heating cycle. When the unmixed slug of cold replenisher chemical comes into contact with the thermistor, a temperature is measured which does not reflect the temperature of the whole chemical. A duty cycle of a heater chosen based on the amount of deviation of such measured temperature from setpoint may provide a heating rate far in excess of that needed considering the temperature of the mass of fluid as a whole. This is especially troublesome where the duty cycle is chosen based on the temperature of a replenisher slug introduced when the chemical as a whole is already at or near setpoint. In such case, the application of too much heat may cause the temperature to overshoot the setpoint target, requiring the consequential activation of one or more, otherwise unnecessary, cooling cycles before the slug is fully mixed and the temperature is again stabilized.